1. Field of the Invention
The present invention relates to a GPS receiver and a portable communication apparatus, and more particularly, is suitably applied to a receiver for the Global Positioning System (GPS) which is used to measure the position of a moving object, for example.
2. Description of the Related Art
Heretofore, in the GPS, a GPS receiver receives satellite signals transmitted from plural GPS satellites, which are circling around the globe, and analyzes the received satellite signals to calculate the distance between each GPS satellite and the GPS receiver itself. Then, it calculates its current position from the calculated distances.
The satellite signal, which is transmitted from a GPS satellite, is a signal which has been subjected to spread spectrum processing with a pseudo noise (PN) code. This PN code is special for each GPS satellite.
Therefore, the GPS receiver can generate a station PN code for every GPS satellite, make the phase of the generated station PN code match that of the PN code of the satellite signal for synchronization, and perform tracking on the satellite signal for synchronous correction. Then, the GPS receiver performs de-spreading processing to demodulate a navigation message (orbit information for calculating a position) from each GPS satellite. As a result, the current position of the GPS receiver can be calculated from the navigation message.
In addition, such a GPS receiver could have a Code Division Multiple Access (CDMA) transmitting/receiving unit to transmit/receive data to/from the base station in the CDMA cellular system based on the Direct Sequence (DS) scheme conformed with the TIA/IS-95 standard in the United States of America, for example, so that the CDMA transmitting/receiving unit can inform the base station in the CDMA cellular system of its current position calculated.
Actually, as shown in FIG. 1, the GPS receiver 1 roughly consists of a GPS receiving unit 1 to demodulate a GPS received signal SI received from each GPS satellite, and a CDMA transmitting/receiving unit 3 to demodulate a CDMA received signal S2 received from a base station in the CDMA cellular system and to transmit a CDMA transmission signal S3, produced by modulating transmission data, to the base station in the CDMA cellular system.
The CDMA transmitting/receiving unit 3 inputs the received CDMA received signal S2 to a CDMA demodulator 4. The CDMA demodulator 4 synchronizes the CDMA received signal S2 to a CDMA demodulation control signal S4 which is supplied from a CDMA control circuit 5, performs the demodulation processing on it, and then outputs the resultant received data S5 to the CDMA control circuit 5.
Further, the CDMA control circuit 5 transmits transmission data S7 such as voices to be transmitted, to a CDMA modulator 6, and also transmits a CDMA modulation control signal S8, which is to control the spread spectrum processing in the CDMA modulator 6, to the CDMA modulator 6.
The CDMA modulator 6 modulates the transmission data S7 in accordance with the CDMA modulating control signal S8 and transmits the resultant signal, which has been subjected to the spread spectrum processing, as a CDMA transmission signal S3 to a high frequency circuit and to the base station in the CDMA cellular system via a CDMA antenna (not shown).
Further, the CDMA control circuit 5 produces a system time signal S6. which is a standard timing of synchronization and demodulation processing, based on the result of demodulation, that is, the received data S5, and supplies it to a GPS control circuit 8 of the GPS receiving unit 2.
The CDMA received signal S2 is produced based on the GPS time which is obtained in such a manner that the base station in the CDMA cellular system receives and analyzes a satellite signal transmitted from the GPS satellite, so that the GPS control circuit 8 can roughly recognize the acquirement of synchronization and the demodulation timing of the demodulation processing for the GPS received signal SI based on the system time signal S6.
As described above, in the GPS receiving unit 2, the GPS control circuit 8 roughly recognizes the acquirement of synchronization and the demodulation timing of the demodulation processing for the GPS received signal SI based on the system time signal S6, and supplies a GPS demodulation control signal S9 based on the demodulation timing to the GPS demodulator 7.
The GPS demodulator 7 synchronizes and demodulates the GPS received signal SI, received from each GPS satellite, based on the GPS demodulation control signal S9 in a short time, thereby making it possible to remarkably reduce power consumption relating to the acquirement of synchronization and the demodulation processing.
Further, the GPS demodulator 7 outputs received data S10 from every GPS satellite, which is obtained by the demodulation processing, to the GPS control circuit 8. The GPS control circuit 8 calculates the current position of the GPS receiver 1 from the plurality of received data S10, which is obtained by the demodulation processing.
Thus the GPS receiver 1 informs the base station in the CDMA cellular system of its current position in such a manner that the current position obtained by the GPS control circuit 8 is passed through the CDMA control circuit 5 as the transmission data S7 and is modulated by the CDMA modulator 6 and the resultant CDMA transmission signal S3 is transmitted to the base station in the CDMA cellular system.
In the GPS receiving unit 2 of the GPS receiver 1 with the above structure, for example, the GPS receiving unit continuously receives the GPS received signal SI as shown in FIG. 2A. And the CDMA transmitting/receiving unit 3 intermittently receives the control signal transmitted from the base station in the CDMA cellular system during the waiting time, until time Ta, continuously receives the control signal from the base station during the line-using time, between time Ta and time Td, and intermittently receives the control signal from the base station again during the waiting time, after time Td.
At this time, the GPS receiver 1 intermittently transmits line connection request data to the base station at predetermined intervals during the line-connection time, between time Ta and time Tb, and continuously transmits the CDMA transmission signal S3 to the base station during the talk time, between time Tb and time Tc.
Therefore, the GPS receiver 1 outputs from the high frequency circuit much stronger radio waves than the received signal S1 which is received by the GPS receiving unit 2, while transmitting the line connection request data and continuously transmitting the CDMA transmission signal S3, which causes a problem in that interference of these radio waves with the GPS received signal S1 remarkably deteriorates reception quality.
Further, in the GPS receiver 1, in the case where the GPS receiving unit 2 intermittently receives the GPS received signal S1 to reduce the power consumption as shown in FIG. 2B, at the same transmission/reception timing in the CDMA transmitting/receiving unit 3 as FIG. 2A, when the intermittent reception timing of the GPS received signal S1, the intermittent transmission timing of the line connection request data during the line connection time, and the continuous transmission timing of the CDMA transmission signal S3 during the talk time coincide with each other, the strong radio waves, which are outputted when the line connection request data and the CDMA transmission signal S3 are transmitted, interfere with the GPS received signal S1, which remarkably deteriorates reception quality.
In view of the foregoing, an object of this invention is to provide a GPS receiver and a portable communication apparatus which allow a satellite signal to be demodulated without deterioration of reception quality.
The foregoing object and other objects of the invention have been achieved by the provision of a GPS receiver and a portable communication apparatus. The demodulation timing of a satellite signal received from a satellite of the global positioning system is controlled in accordance with transmission timing of a transmission signal to a base station in a predetermined communication system, which prevents radio waves which occur in transmission of the transmission signals from interfering with the satellite signal and allows the satellite signal to be demodulated correctly. Thus, the satellite signal can be demodulated without deterioration of reception quality.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.